Experimental and Theoretical Investigations of Temporary Negative Ions

Two experimental techniques and three levels of theory are used to study various aspects of temporary anions, including their energies, lifetimes, and decay channels. Electron transmission (ET) spectroscopy is used to determine the electron affinities (EA's) of two nonconjugated, cyclic polyenes. The ET measurements indicate that the splittings between the pi^* orbitals in these compounds are small. It is shown with the assistance of ab initio calculations that both through-bond and through-space interactions are sizable in these compounds, but that these two interactions oppose one another, resulting in small net splittings. A simple perturbation molecular orbital model is presented, which accounts for the trends in the ionization potentials and EA's. Electron energy loss spectroscopy is used to determine the electron energy dependence of the cross sections for production of the rm 1^3B_ u, rm 1^3A_ g, and rm 1^1B_ u excited states as well as for the excitation of three quanta of CH stretch and of three quanta of C=C stretch in the rm 1^1A_ g ground state of 1,3 -butadiene. The cross sections display peaks due to the two pi^* anion states as well as to various pi^{-1 }(pi^*)^2 core-excited anion states. Evidence for a broad sigma ^* anion state and for pi^{-1}(3s)^2 and pi^{-1}(3p)^2 Feshbach resonances is presented. A new variant of the stabilization method is described and used in conjunction with an analytic continuation procedure to calculate the energies and lifetimes of the lowest energy temporary anion states of N_2 and Mg. This same method is applied at the Koopmans' theorem and single excitation Cl level of theory to the pi^* anion states of 1,4-cyclohexadiene. The calculations gave conclusive evidence for the anion state ordering ^2A_ u below ^2B_{2g} . Finally this method is applied to the chloromethanes, providing definitive assignments of the C-Cl sigma ^* anion states.